Photochromic (phototropic) glasses had their genesis in U.S. Pat. No. 3,208,860. That patent disclosed glasses demonstrating photochromic behavior, i.e., the glasses darken when exposed to actinic radiation, commonly ultraviolet radiation, and return to their original state when the radiation is removed. Such glasses are produced by developing therein crystallites of a silver halide selected from the group of AgCl, AgBr, and AgI. The preferred base glasses are stated to reside in the R.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO.sub.2 composition system.
Subsequent to that fundamental disclosure, intensive continuous research has been carried out to design glass compositions manifesting ever improved photochromic properties. The most extensive commercial application of photochromic glass has been in the field of spectacle lenses, both as prescription lenses and as non-prescription sunglasses. In view of that situation, research has focused principally on developing glasses which darken to a low luminous transmittance; which fade very rapidly to their original state; and which retain those darkening and fading characteristics over the range of temperatures commonly encountered by the wearer of spectacle lenses.
In recent years, organic plastic, non-photochromic lenses have made substantial inroads into both the prescription and non-prescription lens markets, principally because of the light weight of such lenses. Thus, the density of the plastic employed in such lenses is considerably less than glass used for the same purpose. Plastic lenses also possess the capability of being readily dyed. This imparts a permanent tint thereto with consequent reduction in transmittance. However, difficulty has been encountered in producing a practical photochromic lens from an organic plastic.
To gain the perceived advantage of lower weight, while enjoying the effect of photochromic behavior, laminated, glass-plastic lenses have been proposed. A number of U.S. patents disclose various laminated, glass-plastic, ophthalmic lens structures. A structure of current interest is disclosed, for example, in Fretz, Jr. U.S. Pat. No. 4,793,703. In general, the lens of that patent comprises a thin, inorganic glass lens, an organic plastic lens and an intermediate adhesive layer.
To achieve the desired weight advantage, the glass element will be as thin as possible consistent with the objective of achieving both satisfactory photochromic behavior and essential strength levels in the lens design. The conventional ophthalmic lens has a thickness of about 2 mm. It would, of course, be desirable to substantially reduce this thickness. However, as the cross section of a glass article is reduced, the transmittance thereof normally increases. Consequently, to be useful in thicknesses less than 2 mm, the composition of the photochromic glass must be designed to darken to a low luminous transmittance in thin cross sections while retaining the characteristics of fast fading and temperature independence.
Kerko et al. U.S. Pat. No. 4,608,349 is specifically directed to meet the need just expressed for a lightweight, photochromic glass lens that meets the requirements for photochromic behavior and strength. That patent prescribes a family of glasses having relatively narrow composition ranges. These ranges, in weight percent, are as follows:
______________________________________ SiO.sub.2 54-58% B.sub.2 O.sub.3 18-22% Al.sub.2 O.sub.3 7-8% Li.sub.2 O 3.75-4.5% Na.sub.2 O 0-1% K.sub.2 O 5.5-7.5% TiO.sub.2 0-2% ZrO.sub.2 2-4.5% Ag 0.20-0.33% Cl 0.30-0.50% Br 0.04-0.12% CuO 0.007-0.012% PbO 0-0.08% Sb.sub.2 O.sub.3 0-0.20% ______________________________________
The glasses have a Li.sub.2 O:Na.sub.2 O molar ratio of at least 9:1, exhibit good photochromic and strengthening capabilities, and have an extremely neutral color in the darkened state. Neutral coloration is stated in the patent to mean a gray color that closely approximates the color of the standard light source, Illuminant C, as defined in terms of the 1931 C.I.E. trichromatic colorimetric system. The glasses are essentially uncolored in the undarkened state, having a luminous transmittance of 88-92%.
Over the years, there has been a continuing interest in developing a photochromic glass that would darken to a brown tint, rather than a neutral gray. DeMunn et al. U.S. Pat. No. 4,043,781 describes a method of developing photochromic glasses that darken to a brown color. The method requires subjecting glasses within a narrow composition range to a controlled heat treatment. Glasses thus produced were marketed under the registered trademark PHOTOBROWN.
Subsequently, it was learned that a variety of specific photochromic glasses could have a brown tint develop upon darkening. This required including in the glass batch one or more additives selected from the group including Pd, Au, SNO.sub.2, Fe.sub.2 O.sub.3, NiO, Er.sub.2 O.sub.3, Nd.sub.2 O.sub.3 and Co.sub.3 O.sub.4. Such glasses are reported in Hares U.S. Pat. No. 4,251,278, Prassas U.S. Pat. No. 4,757,034 and Grateau et al. U.S. Pat. No. 5,023,029. However, these glasses were designed for use in standard, all-glass, 2 mm thick lenses. A glass lens in accordance with the Hares patent is currently marketed under the registered trademark PHOTOBROWN EXTRA.
There remains a need for a thin photochromic lens of about 1 mm thickness that will darken to a brown color, and that can be used as the glass lens element in the ophthalmic lens of the Fretz, Jr. patent. Stated otherwise, the need is for a lightweight, photochromic lens, as described in the Kerko et al. patent, that will darken to a brown color.
A basic purpose of our present invention is to meet that need. A byproduct is a new photochromic glass that is capable of functioning as a brown sunglass.